Hearthstone's next expansion, The Boomsday Project, is packing a lot of new mechanics into its 135 new cards. Like all new expansions, that means it's going through a lot of play-testing, but one card type in particular needed more attention than most. The powerful spell "projects" were a particularly tricky card type to build and balance.

Projects are spells with symmetrical effects. Instead of just hitting the caster with a positive impact or their opponent with a negative one, it grants both players the same benefit. The idea is to build your deck to make sure you take the best advantage of the project effect, as opposed to your opponent who won't have optimized their deck around it.

"I like to think of projects as kind of like the science projects that you did in school," engineer Cat Morgan told GameSpot. "So there's a team and of course one person always does the work, but you want to get the most benefit from the project, so you want to get that good grade. When people play projects, they really need to think about, 'Am I going to get more benefit from it right now than my opponent might?'"

So far only two have been revealed. The Druid's Biology Project, which gives both players two Mana to ramp up to the limit of 10 that much faster; and the Warlock Demonic Project, which turns a random minion into a demon. The latter is expected to be a combo breaker for facing off against decks like Shudderwock Shaman that depend on one or two key minions to pull off.

"Druid doesn't have a lot of ways to catch up once they're behind," said lead designer Mike Donais. "Biology Project certainly is a risky card though. It's accelerating a class that can maybe take advantage of it the best because they have very expensive cards, like Ultimate Infestation and big minions. But there are gonna be some match-ups where the opponent also has really expensive cards and then they're gonna be certainly paying the cost here, and they're gonna be down a card from playing Biology Project."

Donais continued, "The project cards, probably of all the different mechanics, got some of the most play-testing. We recently hired another play tester, Keaton, or 'Chakki' Keaton Gill. He's helping us out there, and that's been great."

Not every class has its own project. Donais said it will be roughly an even split between Projects and another new card type--Omega cards that grant big effects if played once you've reached 10 Mana.